This invention relates to a primary nonaqueous electrolyte electrochemical battery cell, such as a lithium/iron disulfide, with good low temperature performance characteristics.
Batteries are used to provide power to many portable electronic devices. Common advantages of lithium batteries (those that contain metallic lithium or lithium alloy as the electrochemically active material of the negative electrode) include high energy density, good high rate and high power discharge performance, good performance over a broad temperature range, long shelf life and light weight. Lithium batteries are becoming increasingly popular as the battery of choice for new devices because of trends in those devices toward smaller size and higher power. The use of high power consumer devices in low temperature environments is also becoming more common. While lithium batteries can typically operate devices at lower temperatures than batteries with aqueous electrolytes, electrolyte systems that provide the best high power discharge characteristics, even after storage for long periods of time, do not always give the best performance at low temperatures.
One type of lithium battery, referred to below as a Li/FeS2 battery, has iron disulfide as the electrochemically active material of the positive electrode. Li/FeS2 batteries have used electrolyte systems with a wide variety of solutes and organic solvents. The salt/solvent combination is selected to provide sufficient electrolytic and electrical conductivity to meet the cell discharge requirements over the desired temperature range. While the electrical conductivity is relatively low compared to some other common solvents, ethers are often desirable because of their generally low viscosity, good wetting capability, good low temperature discharge performance and good high rate discharge performance. This is particularly true in Li/FeS2 cells because the ethers are more stable than with MnO2 cathodes, so higher ether levels can be used. Among the ethers that have been used are 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DIOX), which have been used together and in blends with other cosolvents. However, because of interactions among solvents, as well as with the electrolyte solutes, cell performance has been difficult to predict based on the properties of individual solvent and solute components.
A wide variety of solutes has been used in Li/FeS2 cell electrolytes; lithium trifluoromethanesulfonate (also commonly referred to as lithium triflate or LiCF3SO3) is among them. An example of a Li/FeS2 cell with a lithium triflate solute in a solvent blend comprising DIOX and DME is found in U.S. Pat. No. 4,952,330, which is hereby incorporated by reference. A solvent blend of 40 to 53 volume percent cyclic ether (e.g., DIOX), 32 to 40 volume percent linear aliphatic ether (e.g., DME) and 8 to 18 volume percent alkylene carbonate (e.g., propylene carbonate) is disclosed. However, such an electrolyte can result in poor cell discharge performance at high discharge rates.
Another example of a cell with an electrolyte containing lithium triflate dissolved in a solvent comprising DIOX and DME is found in U.S. Pat. No. 5,290,414, which is hereby incorporated by reference. A blend of from 1:99 to 45:55 DIOX:DME with an optional cosolvent (e.g., 0.2 weight percent 3,5-dimethylisoxazole (DMI)) is disclosed as a solvent. The disclosed cell had low impedance following storage at high temperature.
While electrolytes containing lithium triflate can provide fair cell electrical and discharge characteristics, such electrolytes have relatively low electrical conductivity, and lithium triflate is relatively expensive. Lithium iodide (LiI) has been used as an improved performance and lower cost alternative to lithium triflate. U.S. Pat. No. 5,514,991, which is hereby incorporated by reference, discloses a cell with improved high rate discharge performance, even after storage at high temperature. LiI is the sole solute, and the electrolyte solvent comprises at least 97 volume percent ether (e.g., 20:80 to 30:70 by volume DIOX:DME, with 0.2 volume percent DMI as a cosolvent).
However, it has been discovered that when LiI is used as the solute in an electrolyte containing DME in the solvent, especially more than 40 volume percent, discharge capacity at low temperatures, such as −20° C. and below, can be very low. This is believed to be due to formation of a DME solvate that can precipitate from the electrolyte solution at low temperatures or otherwise degrade low temperature cell performance. Simply reducing the DME content in the solvent can prevent this problem, but some of the improvement in high rate and high power discharge performance realized with LiI as the solute is sacrificed.
In view of the above, an object of the present invention is to provide a primary nonaqueous electrolyte battery with good discharge characteristics at low temperatures.
Another object is to provide a Li/FeS2 cell with excellent electrical and discharge characteristics, including high rate and high power discharge capacity, over a broad temperature range, including low temperatures.
Another object of the invention is to provide a cell that is economical to produce and has excellent electrical characteristics during and following storage and use at low temperatures.
Yet another object of the invention is to provide a 1.5 volt primary nonaqueous electrolyte cell with a LiI-containing electrolyte with good low temperature electrical performance.